The present invention relates to an instrument for measuring contact lens parameters and more particularly to an instrument for accurately measuring contact lenses while immersed in a solution of known refractive index, e.g., saline solution.
The soft contact lens industry, as well as doctors prescribing soft contact lenses, have been confronted with an inability to accurately determine the essential parameters of soft contact lenses after hydration. Previously, most soft contact lens parameters were determined by the industry while the lens was in the "hard" or unhydrated state. The values thus determined are subsequently converted to "expected parameters" based upon calculated changes occurring during hydration. While in the hard stage, the contact lens may be measured using conventional instruments for measuring hard contact lenses.
The practice of determining parameters of a soft contact lens by determinations made while it is in the unhydrated or "hard" stage is fraught with disadvantages. Most of these disadvantages result from unpredictable and inconsistent variations between lenses when hydrated. Since soft contact lenses rapidly lose water of hydration and are extremely flexible and susceptible to distortion when in the hydrated state, instruments for measuring hard contact lenses cannot accurately determine the parameters of a hydrated soft contact lens. For example, dial indicators which are normally used to determine the thickness of a hard contact lens or an unhydrated soft contact lens, but they compress a hydrated soft contact lens producing inaccurate results. Also the index of refraction change with the water content, and power measurements are inaccurate and unreproducible unless the soft lens is immersed.
There exist some devices for approximating the parameters of hydrated soft lenses but these are highly dependent on operator skill, the operator's ability to interpolate and the assumption that ordinary focimeters can accurately measure the power of an immersed lens.
As explained in an article by L. Wray, the Ophthalmic Optician, Apr. 1, 1972, pgs. 256, 261-264 and Apr. 15, 1972, pgs. 301-304 and 309-311, the relatively simple formula for vertex power (front and back) used for air-lens interfaces will not provide the requisite accuracy of calculated vertex power for an immersed lens. The lens thickness and the power of each surface are essential factors in accurately measuring immersed hydrated contact lenses. Without accurate values for these factors, the power computation may have an error greater than 0.5 diopter. The problems of obtaining accurate values for thickness, radii and power are also discussed in the article.
U.S. Pat. No. 3,804,523 relates to a conventional radiuscope for measuring the thickness of hydrophilic contact lenses using a special lens support. The lenses are not immersed during measurement.
U.S. Pat. No. 3,985,445 relates to an optical instrument for measuring the power of an immersed hydrophilic contact lens. Lens power is determined by a scale showing the position of a target projected along the optical axis through the lens to a reference plane in an observation system. This patent erroneously teaches the calculation of lens power using the same formula used to calculate lens power when the lens is in air. As discussed hereinabove, lens thickness and anterior surface radius are necessary for an accurate determination of lens power when the lens is measured while immersed in a liquid.
It is an object of the present invention to accurately measure the parameters of an immersed, hydrated, soft contact lens.
It is another object of the present invention to overcome the disadvantages of the prior art devices.
It is still another object of the present invention to provide a single device for accurately measuring back vertex power, thickness and posterior lens radius of hydrated soft contact lenses.